1. Field of the Invention
The present invention relates to chemical characterization in harsh and chemically reactive environments, and more specifically, it relates to a specialized optical probe that provides spectral signals free from interfering signals found in currently available probes.
2. Description of Related Art
Induced radiative effects such as Raman scattering and fluorescence have become extremely valuable tools associated with the non-destructive determination of molecular constituents. To characterize a composition in a remote or hostile environment, optical fibers may advantageously be used to deliver excitation energy to a sample under investigation and to carry scattered radiation back to means for spectral analysis. An excitation source path may take the form of a laser providing a stimulus at an appropriate wavelength coupled to an input fiber, and a collection path may be made up of a second fiber carrying return radiative information to a spectral analysis tool such as a spectrograph.
Such remote spectral analysis presents technical challenges, however, including the strong scattering signature of the material used for the optical fiber, this interference potentially being generated by both the laser excitation in the illumination fiber and any strong Rayleigh (unshifted) Scattering allowed to enter the collection fiber. These spurious fiber signatures can compete with, or even overshadow, the desired signature of the sample under test, particularly when long lengths of fiber are used.
In U.S. Pat. No. 5,377,004, a narrowband reflective element not only folds the laser energy into a common sample illumination/collection optic, but also serves to reject a substantial portion of the Rayleigh scattering received from the sample. Since the reflection of a narrow band of wavelengths is much easier to control and improve than with transmission, this arrangement simultaneously reflects significantly more laser excitation light than a transmission element will pass, while transmitting more of the scattered signal than a transmission element will reflect, particularly those signals close to the excitation wavelength.
Additionally, folding of the illumination energy into an in-line collection path facilitates the use of a dispersive element, preferably another holographic optical element, in the illumination path to remove spurious scattering generated within the fiber from the excitation source. Such a highly efficient dispersive filtering element cannot be used in a configuration having the relatively weak spectra of interest folded out of an in-line illumination path. The use of a dispersive element also allows the use of spatial filters which may take advantage of apertures in the form of pinholes or slits in the illumination or sample path as a means to remove all but the laser energy.
A holographic notch filter may be inserted into the collection path between the narrowband reflective element and the return optical fiber, this notch filter being operative to further remove Rayleigh scatter from the scattered spectra. Additionally, beam redirection means may be provided so that the illumination and collection fibers may be substantially parallel to one another at their interface to the probe proper, thereby resulting in a more compact assembly. Preferably, a holographic transmission grating is utilized as this beam-redirecting device in the illumination path, thereby performing an additional function of excitation prefiltering before the primary filtering performed by the narrowband reflective element.
In other embodiments, a holographic beam splitter may be used as the means to redirect the illumination radiation, thus providing an alternative approach to laser light prefiltering. In further alternative embodiments, a holographic notch filter may be used as the narrowband reflective element, in which case the illumination path redirection means may take the form of another holographic notch filter or a holographic transmission or reflection grating. Mirrors or prisms may also be used advantageously in the illumination path, depending upon the desired final geometry.
In order to observe a process flow as opposed to direct sample illumination, a light-conductive element may be added between the probe and the environment under characterization, be it liquid, gaseous, plasma, etc. Preferably, this element is composed of a material which does not produce an unwanted signature, for example a nonsilica material such as fluorite.
U.S. Pat. No. 5,112,127 describes a fiber-optic probe which is useful for measuring Raman spectra of samples remote from the light source and detector. The probe head contains optical components which selectively remove unwanted fluorescence and Raman scattering arising from the interaction between the Raman excitation source radiation and the input optical fiber. The optics also filter the Raman excitation source into a return optical fiber leading to a spectrometer or detector. In one embodiment, the disposition of optical components provides a compact probe geometry with parallel input and output fibers at one end and a sampling port at the other end. An encasement for the optics is also disclosed, for sealing the components against the environment, and for coupling the probe to specialized sampling attachments, such as for conducting Surface Enhanced Raman Spectroscopy.
The probe is used as a component in instrumentation comprising 1) a light source, such as a laser which is coupled to an optical fiber or fiber bundle for the low-loss transmission of the light to the probe, which is placed in contact with the sample to be measured, and 2) a return fiber or fiber bundle exiting the probe which returns the light scattered from the sample to a spectroscopic analyzing instrument such as a spectrometer. The probe disclosed in this invention consists of 1) optics for filtering the excitation light, thus removing interfering Raman scattering and fluorescence arising from the excitation fiber; 2) optics for focusing the excitation light onto a sample external from the probe; 3) optics for collecting the scattered light from the sample and removing the intense laser excitation line from the desired spectral features; 4) optics for refocusing the scattered light for acceptance by the exit fiber or fiber bundle. An enclosure, ideally sealed from the environment, containing the optical components, is also a feature of the invention.